The reproduction of electronic video images involves the transmission of a waveform known as a video signal. Various types of video signals, such as for example composite video, S-video, and component video, have certain shared characteristics. In general, the video signal includes both video image display information and associated synchronizing information. A graphical representation of a generic video signal 110 known in the arts is shown in FIG. 1. As shown in the example, the video information is generally in the form of a signal having a black reference level 112, and a higher level representing white 114. The continuum of levels 116 between the black level 112 and the white level 114 is then used to represent degrees of gray. The synchronizing information used in formatting the signal display includes vertical and horizontal display alignment and color decoding information. The lowest value of the synchronizing signal is referred to as the “sync tip” 120. The synchronizing pulses 118 are positioned in portions of the signal 110 that do not contain video display information, that is, below the black level 112 to prevent disruption of the video image display. This is referred to as the “blank level” 124. The synchronizing pulses 118 shown in this example have a reference level at the blank level 124.
The video signal waveform 110 from the sync tip level 120 to the white level 114 used in this example has a nominal peak-to-peak amplitude of 1V. The “front porch” 130 refers to the portion of the video signal 110 that occurs between the end of the active video interval 128 and the falling/leading edge of the horizontal sync pulse 118. The “back porch” 126 refers to the portion of the video signal 110 that occurs between the rising/trailing edge of the horizontal sync pulse 118 and the beginning of the active video interval 128.
In video signal processing, automatic level control (ALC) is commonly used to maintain the offset level or brightness of the video signal. In a typical ALC implementation, the offset applied to the video signal is automatically adjusted until the difference between the target back-porch level (also known as blanking level 600) and the measured back-porch level is driven to zero at the output of the video system. Similarly, automatic gain control (AGC) is commonly used to maintain the amplitude or contrast of the video signal. In a typical AGC implementation, the gain applied to the video signal is automatically adjusted until the ratio of the target sync-height to the measured sync-height is driven to unity. One problem encountered in the arts is that the back-porch level referred to the input of the gain control stage is typically not guaranteed to be zero. Therefore, a step change in the gain applied to the video signal can cause a corresponding step change in the back-porch level referred to the output of the video system. Restoring the desired back-porch level after a step change in the gain may require multiple iterations of a recursively filtered ALC algorithm. The net effect is the appearance of an offset transient whenever there is a step change in the gain.
Offsets may be applied to either the coarse analog offset stage or the fine digital offset stage of the signal processing system. Similarly, gains may be applied to either the coarse analog gain stage or the fine digital gain stage of the signal processing system. Assuming for the purpose of clarifying and simplifying the description, that both the coarse analog offset and the coarse analog gain remain constant, the fine digital offset and fine digital gain may be discussed individually. The same approach toward adjusting the offset may alternatively be applied to either the coarse analog offset or the fine digital offset, or to both.
It is known in the arts to make adjustments to an offset control value by finding the difference between a desired target back porch value and an actual measured back porch value. The difference may be multiplied by a filter coefficient, and the result added to the immediately preceding fine offset control value to obtain a new fine offset control value. Herein denominated the “classical approach,” this technique may be expressed,OF[n]=OF[n−1]+α*(LNOM−NBP)/Gp  [Equation 1],where OF[n] is a new fine offset control value, OF[n−1] is the immediately preceding fine offset control value, α is a first-order recursive filter coefficient, LNOM is the desired or target back porch level referred to the signal output, NBP is the mean measured back porch value and Gp is the gain applied between a fine offset stage and the signal output where the back porch level is measured.
A problem with the classical approach to automatic level control (ALC) is that changes in the gain applied to the video signal, for example by an AGC technique, can cause offset transient. Offset transient occurs when a relatively large gain change, in this case fine gain, is made in the video signal processing system with the result that the required new fine offset control value OF[n] is dramatically different from the immediately preceding fine offset control value, OF[n−1]. Since the classical approach does not consider gain when setting the new fine offset control value OF[n], it can take numerous iterations of the commonly used ALC techniques to arrive at a desirable new fine offset control value OF[n]. These difficulties are still more acute when the ALC is applied relatively infrequently, such as at the frame rate, making signal anomalies more problematic than they might be if occurring at shorter intervals, such as at the line rate.
FIG. 2 (prior art) and FIG. 3 (prior art) help to illustrate the problem of offset transient using the classical approach. Assume, for the purposes of illustration, an arbitrary step gain change in a signal processing system from 1.25 to 1.0 (y-axis), as shown FIG. 2. Further assume that the classical ALC approach is applied at a frame rate. Plotting the change in the target back porch level over the difference of the mean measured back porch level and the synch tip level, (LNOM/(NBP−NST), shows that the change can be accomplished in relatively few frames (x-axis), the bulk of the transition being completed within about 40 frames. Referring now to FIG. 3, the difference between the target back porch level and the measured back porch level (LNOM−NBP) is plotted relative to the number of frames (x-axis). It can be seen that the gain change depicted in FIG. 2 results in a sudden and dramatic departure of the measured back porch level from the target back porch level. Moreover, it takes a significant number of frames, i.e., iterations of the classical ALC approach in this example, to reach the desired target back porch level.
Due to these and other problems, it would be useful and desirable in the arts to provide improved ALC methods and systems capable of providing quickly converging and accurate automatic level control dynamically adjusted for applied gain.